1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine or a laser beam printer, and a method for controlling the image forming apparatus.
2. Description of the Related Art
Generally, an image forming apparatus forms an electrostatic latent image by exposing the surface of a photosensitive drum that has been uniformly charged by a charging unit, according to image data, and forms a toner image by a developing roller supplying toner and developing the electrostatic latent image formed on the photosensitive drum. Furthermore, image formation is performed by a transfer roller transferring the toner image formed on the photosensitive drum onto a printing material. Note that the developing process on the photosensitive drum is performed in a state in which the developing roller is in contact with the photosensitive drum. Further, in the case in which images are continuously formed on a plurality of printing materials, the image forming apparatus continuously performs the image forming process while maintaining a state in which the developing roller and the photosensitive drum are in contact, even during the period between one printing material being conveyed and the next printing material being conveyed (hereinafter, referred to as “the interval between sheets”), in order to maintain throughput.
In an image forming apparatus such as mentioned above, a rendering process on image data for forming an image on a printing material may require a longer time than normal. For example, a longer time may be required for a rendering process on image data in the case of a high-resolution color image or photographic image. In such cases, the image forming apparatus postpones the timing for starting image formation by expanding the interval between sheets. However, in the case in which the interval between sheets is expanded by extending the start timing of image formation, there is the problem of the degradation speed of the photosensitive drum being accelerated compared with normal continuous image formation, since the charging unit continues applying a charging voltage to the photosensitive drum during the time for the expanded interval between sheets.
As a technique for solving this problem, for example, Japanese Patent Laid-Open No. 2006-285294 proposes an image forming apparatus that temporarily stops charging the photosensitive drum if the interval between sheets is expanded more than normal, due, for instance, to an image rendering process performed by a controller unit taking a long time. In the image forming apparatus disclosed in Japanese Patent Laid-Open No. 2006-285294, the degradation speed of the photosensitive drum is thereby slowed.
However, the above-mentioned conventional technique has the following problem. For example, with the image forming apparatus disclosed in Japanese Patent Laid-Open No. 2006-285294, contact between a photosensitive drum and a developing roller is maintained during the interval between sheets, despite a charging voltage having been stopped. Therefore, there is the problem of the wear of the photosensitive drum and the developing roller being accelerated due to friction therebetween.